Model to aid study of river flooding

The Army Corps of Engineers has released a long-awaited computer model of the Mississippi River.

The model, which mimics river behavior over a 321-mile stretch between Thebes, near the southern tip of Illinois, and Keokuk, Iowa, will have a wide variety of uses, experts say — especially in looking at how flooding occurs.

“There’s two parts of the model,” said Ted Labelle, an engineer for the Springfield-based Crawford, Murphy & Tilly. “One is to create the topography of the ground as it exists and once you have that then you send send some flows down it that the river would have.”

In effect, Labelle said, by pouring a simulated amount of water into the replicated Mississippi River channel, modelers will be able to determine how high the water will rise under a variety of conditions. Labelle said his clients, a Missouri group called Neighbors of the Mississippi, are eager to apply the model to see how significant the impacts of overbuilt levees are.

As previously reported, more than a dozen levees north of Grafton have been raised from 2 of 4 feet above their legally authorized height, creating concerns that they might displace floodwaters onto their less-protected neighbors.

“We can also look and see what happens if other people start raising their levees,” Labelle said. “We could see what happens if a person raises their levee a certain number of feet, and how much it would impact us.”

Scott Whitney, district flood risk manager with the Corps’ Rock Island division, said the model was the work of about a dozen experts across several government agencies and cost close to $500,000 to complete. It took 17 months to develop. The model is so complex, it can take hours for a scenario to run.

According to Whitney, the model consists of hundreds of cross sections of river channel that create the channel profile through which the water flows. Bank elevations are also included, and levees are built into the model data, and the model takes into account how the water interacts with each. In the event that the water gets high enough to overtop a levee, the model simulates the impact the failure has on water levels, as massive flows divert into the failing levee district.

The Mississippi is not the only stream built into the model — major tributaries such as the Illinois and Missouri rivers also have sections incorporated so their discharge into the Mississippi can be factored.

Labelle said he was impressed by how close the model matched what has been observed in reality. The Corps calibrated the model by running four recorded flood events — 2008, 2013, 2014 and 2017 — through it, and working to ensure they reflected observations. The 1993 flood was not used in the calibration process, due to lack of sufficiently detailed data.

“I think is very important that they took four or five historic floods, and modeled it on the computer as to what the river stages would be given the existing conditions,” Labelle said, “and then they were able to compare water level elevations that were computed to actual water elevations that there are records of, and obviously the better they matched, the better the model is.”

While Corps officials initially planned to run some simulations, including one comparing the overbuilt levees to the heights they are supposed to be at, officials ultimately decided just to release the model for public use. According to Whitney, this was done in part to prevent any one simulation from overshadowing the model.